The remarkable paternal apparatus of the goose, and a note on animal genitalia

This is a family-friendly website, but that doesn’t mean we can ignore the biological oddities surrounding reproduction. Here is one, courtesy of a Twitter post (I can’t bring myself to say “tweet”) by SciCurious, passed on by Matthew Cobb. It shows the collection of semen from a goose.

I’m not sure why they’re doing this, which involves manually stimulating the male at great length and then collecting his semen in a cup. It must be for breeding purposes, but I wasn’t familiar with this in geese. Artificial insemination is regularly done in turkeys, for they’ve been bred to have such large breasts that the males can no longer mount the females during copulation. This is one of the more deleterious effects of artificial selection! But geese?

Anyway, the real action in this video happens at about 1:03, when after much stimulation the penis everts and ejaculates. Notice its corkscrew shape and spines.

I’m convinced that features like these evolve via sexual selection: the female “perceives” through tactile stimuli that this is a genital she likes. For more on this, read William Eberhard’s remarkable but underappreciated book Sexual selection and Animal Genitalia. It’s one of my favorite evolution books, for it calls attention to one of the few rules of evolutionary biology: if species are very similar in appearance, but there is only one good way to tell them apart, that difference is almost invariably in the shape of the male genitalia.

This holds not only in birds, but many other groups as well. It’s very common in insects, and one of the first things insect taxonomists look at is the male genitals

Tellingly, in spiders the diagnostic feature is not the male genitals, but the pedipalps, a pair of anterior appendages that take the sperm from the genitals and transfer them to the females. Since the females actually contact the pedipalps but not the genitals, this supports the notion that the female’s tactile perception of male sperm-transfer organs has caused those organs to evolve rapidly and differentially among related species. Species-specific differences occur not just in penis shape or pedipalps, but also in organs involved with copulation, such as the “claspers” in male fruit flies. Here are some differences between the penises of closely-related damselflies:

My own guess for the evolutionary process that has operated here is sexual selection based on pre-existing female preferences that have nothing to do with the quality of the male: this is called the “sensory bias” hypothesis. Another suggestion is that the genitals of related species diverge so they can tell each other apart (the “species recognition hypothesis”): a female “feels” when she’s being rogered by the wrong male, and can terminate copulation prematurely. But I doubt that this explanation is correct because genitals also diverge rapidly in animals that don’t coexist with close relatives (as in isolated species on islands), so there’s no evolutionary pressure to tell your own males from those of a closely related species.

Anyway, on to the goose:

You’re probably asking yourself: do all birds have penises like this? The answer is no—most birds don’t have penises at all but cloacas: holes that serve for sperm exit and (in females) entry, as well as for excretion in both sexes. When birds copulate, they merely put their cloacas together (this is known romantically as the “cloacal kiss”), and insemination occurs very quickly.

But some birds, including geese, ducks, ostriches, and other ratites (large flightless birds), have “penises”, which aren’t homologous to the mammalian penis but are eversions of the cloacal wall erected by the pumping of lymph rather than blood (as in mammals). I’m sure I’ve posted this before, but here’s a remarkable bird “penis,” that of the Argentine lake duck (Oxyura vittata), whose paternal apparatus can be more than 42 centimeters (17 inches!) long. It is the longest penis relative to body size of any vertebrate.

18 Comments

Where did you get the information that flamingos have penises? It’s new to me, and isn’t mentioned in either of the sources I’ve just looked in and would have thought were authoritative, Montgomerie & Briskie 2007 and del Hoyo 1992.

According to Montgomerie & Briskie, there’s only one genus of birds outside of Gallanserae and Palaeognathae that has an intromittent organ, and it’s a parrot.

Before my fat fingers and the predictive text on my phone screwed that up, I was asking about the intromittent organs *of the birds*. I can’t remember what the latest DNA phylogeny of birds suggested for these groups (ie Gallanserae, Palaeognathae, and this odd/lucky parrot). Do you (or anyone else) know? Is this convergent evolution, or did the avian schlong-ette only evolve once?

If we were to optimize schlongs as a binary presence/absence character onto bird phylogeny, we would find them to be present in the common ancestor, their loss to be a synapomorphy of Neoaves, and a single case of reversal in that one genus of Madagascar parrots. (They were also lost, or at least greatly reduced, one or more times within Galliformes.)

But seriously, while I have no problem fathoming how a cloacal derivative could evert itself under hydrostatic pressure, just how does the dang thing retract? It just doesn’t look like simple pressure relief would be sufficient.

Why do you favor the hypothesis “the female “perceives” through tactile stimuli that this is a genital she likes” over the (non-mutually exclusive) hypothesis that the evolution of the genitalia is a result of competition — between different males (that the ridges and spines etc. have evolved to displace the sperm of other males) as well as conflict between males and females (where the female tries to control which males fertilize her eggs; ducks are a bird species with forced copulation, and the convoluted female reproductive tract can prevent males from successfull fertilization).

For an example, Patty Brennan from Yale had a paper in ProcRoySocB (doi: 10.1098/rspb.2009.2139).

The competition hypothesis doesn’t really explain species differences (presumably there’s an optimal sperm-removing morphology in one group), but, more important, such differences are seen, as I think Eberhard notes, in species that mate only once, so the females have no basis for comparison and there is no sperm competition. Finally, the differences in “genitalia” are seen in aspects of the genitals, such as the “claspers” of Drosophila, that have nothing to do with sperm removal, for the are not intromittent.

“If species are very similar in appearance, but there is only one good way to tell them apart, that difference is almost invariably in the shape of the male genitalia.”

A sort of inverse of this holds in orchids that practice pseudocopulation– the most important taxonomic clue in Lepanthes orchids is the fake female insect genitalia, called the “appendix” under the lip of the orchid. The appendix precisely matches the male genitalia of the insect species which pollinates it.

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